This study seeks to understand the compositional details of N-containing
aromatic compounds (NACs) emitted during biomass burning (BB) and their
contribution to light-absorbing organic carbon (OC), also termed brown carbon
(BrC). Three laboratory BB experiments were conducted with two United States pine
forest understory fuels typical of those consumed during prescribed fires.
During the experiments, submicron aerosol particles were collected on filter
media and subsequently extracted with methanol and examined for their optical
and chemical properties. Significant correlations (p<0.05) were
observed between BrC absorption and elemental carbon (EC)∕OC ratios for
individual burns data. However, the pooled experimental data indicated that
EC∕OC alone cannot explain the BB BrC absorption. Fourteen NAC formulas were
identified in the BB samples, most of which were also observed in simulated
secondary organic aerosol (SOA) from photooxidation of aromatic volatile organic compounds (VOCs) with
NOx. However, the molecular structures associated with the identical NAC
formula from BB and SOA are different. In this work, the identified NACs from
BB are featured by methoxy and cyanate groups and are predominately
generated during the flaming phase. The mass concentrations of identified
NACs were quantified using authentic and surrogate standards, and their
contributions to bulk light absorption of solvent-extractable OC were also
calculated. The contributions of identified NACs to organic matter (OM) and
BrC absorption were significantly higher in flaming-phase samples than those
in smoldering-phase samples, and they correlated with the EC∕OC ratio
(p<0.05) for both individual burns and pooled experimental data, indicating that
the formation of NACs from BB largely depends on burn conditions. The average
contributions of identified NACs to overall BrC absorption at 365 nm ranged
from 0.087±0.024 % to 1.22±0.54 %, which is 3–10 times higher than
their mass contributions to OM (0.023±0.0089 % to 0.18±0.067 %), so the NACs with light absorption identified in this work from BB
are likely strong BrC chromophores. Further studies are warranted to identify
more light-absorbing compounds to explain the unknown fraction (>98 %) of BB BrC absorption.

Polycyclic aromatic hydrocarbons (PAHs) and their derivatives are typical
BrC chromophores (Samburova et al., 2016; Huang et al., 2018), of
which the light absorption in the UV and visible wavelength range is highly
dependent on ring numbers and the degree of conjugation (Samburova et al.,
2016). However, PAH emissions are not source-specific but are associated
with multiple different combustion processes, including BB (Samburova et
al., 2016), coal burning (Chen et al., 2005), and motor vehicle emissions
(Riddle et al., 2007). Therefore, PAHs are not unique to BB BrC.
N-containing aromatic compounds (NACs) are another class of BrC chromophores
that have been detected in BB (Lin et al., 2016), cloud water (Desyaterik et
al., 2013) and atmospheric particles (Zhang et al., 2013; Teich et al.,
2017). In water extracts of atmospheric particles, NACs can contribute
more than 3 % of the light absorption at 365–370 nm (Zhang et al.,
2013; Teich et al., 2017). These results suggest that NACs are important BrC
chromophores, but their composition and structures are less certain for BB
aerosols. Nitrophenols, nitrocatechols, and methyl nitrocatechols (including
isomers) are commonly observed in BB aerosols (Iinuma et al., 2010; Claeys
et al., 2012; Lin et al., 2016, 2017) and are also generated from
the photooxidation of benzene, toluene, and m-creosol in the presence of
NOx (Iinuma et al., 2010; Lin et al., 2015; Xie et al., 2017a). As such,
other NAC structures specific to BB are needed to represent BB BrC
chromophores. Additionally, very few studies have examined the influence of
burn conditions on the formation of NACs in BB emissions, although it is
well known that increasing combustion temperature, or flaming-dominated
combustion, is associated with strong BrC absorption (Chen and Bond,
2010; Saleh et al., 2014).

The present study attempts to characterize the compositional profile of NACs
from BB, identify additional NAC structures in laboratory BB samples, and
evaluate the contributions of NACs to bulk absorption of solvent-extractable
OC from BB. A high-performance liquid chromatograph interfaced to a diode
array detector (HPLC/DAD) and quadrupole (Q) time-of-flight mass
spectrometer (ToF-MS) was used to examine NACs in PM2.5 (particulate
matter with aerodynamic diameter ≤2.5µm) from three BB
experiments. A thermal–optical instrument determined bulk OC and elemental
carbon (EC) in the PM, and a UV–visible (UV–Vis) spectrometer was used to measure total
BrC absorption in methanol extracts of BB PM2.5. In this work, a number
of NAC formulas with structures that might be specifically related to BB
were identified, and the contributions of identified NACs to bulk BrC
absorption were calculated. These results shed light on the light-absorbing
characteristics of BB OC at bulk chemical and molecular levels, benefiting
the understanding of BrC sources and chromophores.

2.1 Laboratory open BB simulations

Laboratory simulations of open BB were conducted at the U.S. EPA (Research
Triangle Park, RTP; North Carolina, NC) Open Burn Test Facility (OBTF), a
70 m3 enclosure, as detailed in Grandesso et al. (2011). Details of the
protocols for biomass fuel collection and burn simulations were provided
elsewhere (Aurell and Gullett, 2013; Aurell et al., 2015; Holder et al.,
2016). Briefly, forest understory fuels were gathered from two different
locations in the southeastern United States – Florida (FL) and NC. The FL
forest field (Eglin Air Force Base, FL) is characteristic of a well-managed
long leaf pine (Pinus palustris) ecosystem. The NC forest was located near the EPA campus
in RTP, and it contained mainly loblolly pine (Pinus taeda) with some deciduous
hardwood tree leaf litter. Biomass fuel was divided by a quartering
procedure (Aurell and Gullett, 2013) and burned in batches (1 kg) on an
aluminum-foil-coated steel pan (1 m×1 m). Ambient air was pulled
into the OBTF through a large inlet at ground level and the combustion
exhaust was drawn through a roof duct near a baghouse using a high-volume
blower. PM2.5 was sampled at 10 L min−1 on Teflon (47 mm, Pall,
Ann Arbor, Michigan, USA) and preheated (550 ∘C, 12 h) quartz filters
(QF, diameter 43 mm, Pall) with a PM2.5 impactor (SKC, Pittsburgh,
Pennsylvania, USA). For the NC forest fire simulation, filter samples were
collected during an initial flaming phase lasting approximately 1–3 min.
After most of the flames were extinguished, a second set of filter
samples were obtained for the smoldering emissions. Smoldering samples were
collected until there was little or no visible smoke being emitted from the
fuel bed, typically lasting 6–15 min. Two separate experiments were
done with the NC forest fuels in spring and summer, respectively, with
different ambient temperatures (Table S1 in the Supplement). Sampling of the FL forest fire
simulations was done in fall over the complete burn and was not divided into flaming and smoldering phases.
Only one experiment was done for the FL forest fuels collected in
fall. Background samples were obtained post-burn inside the OBTF. A summary
of the sample information is provided in Table S1.

2.2 Bulk carbon and light absorption measurement

Details of the bulk OC, EC, and light absorption analysis methods are
provided in Xie et al. (2017a, b). Briefly, the bulk OC and EC were
measured using an OC–EC analyzer (Sunset Laboratories, Portland, OR) with a
modified NIOSH method 5040 protocol (NIOSH, 1999). For light absorption
measurement, one filter punch (1.5 cm2) was extracted in 5 mL methanol
(HPLC grade) ultrasonically for 15 min and then filtered through a
30 mm diameter polytetrafluoroethylene (PTFE) filter with a 0.2 µm pore
size (National Scientific Company). The light absorption of methanol
extracts was measured with a UV–Vis spectrometer (V660, Jasco Incorporated,
Easton MD) over the wavelength range of 200 to 900 nm. To ensure data
quality, the wavelength accuracy (±0.3nm) and repeatability (±0.05nm) were tracked every month with a NIST traceable holmium oxide
standard. Solvent background was subtracted with a reference cuvette
containing pure methanol. The extracted filter was air-dried in a fume hood
overnight, and the residual OC was measured with the Sunset thermal–optical
analyzer. The extraction efficiency (η, %) of OC by methanol is
calculated by

(1)η=OCb-OCrOCb×100%,

where OCb is the OC content of the PM2.5 filter before extraction and
OCr is the OC content in the air-dried filter after extraction.

The light absorption coefficient of the methanol extracts (Absλ,
Mm−1) is calculated as

(2)Absλ=(Aλ-A700)×VlVa×Lln(10),

where A700 is subtracted from Aλ to correct baseline drift,
Vl (m3) is the solvent volume (5 mL) used for extraction, Va
(m3) is the air volume of the extracted filter area, L (0.01 m) is the
optical path length, and ln (10) converts the absorption coefficient in
units of m−1 from log base 10 to natural log (Hecobian et al., 2010).
The bulk mass absorption coefficient (MACλ, m2 g C−1)
is calculated by

(3)MACλ=AbsλCOC,

where COC is the mass concentration of extractable OC
(OCb−OCr) for each filter sample (µg m−3). The solution
absorption Ångström exponent (Åabs) is determined from the
slope of the linear regression of log10(Absλ) vs. log10(λ) over the λ range of 300 to 550 nm. In the
current work, Absλ and MACλ were focused at 365
and 550 nm, representing the BrC absorption at the near-UV and visible
regions, respectively (Zhang et al., 2013; Saleh et al., 2014). The EC∕OC ratio,
methanol extraction efficiency (η), and light-absorbing properties
(Absλ, MACλ, and Åabs) of each BB sample are
listed in Table S1 in the Supplement and summarized in Table 1.

2.3 Filter extraction and HPLC/DAD-Q-ToF-MS analysis

The PM2.5 filter extraction and subsequent instrumental analysis
methods used here are the same as those described in Xie et al. (2017a).
Briefly, a 4–6 cm2 piece of each filter was pre-spiked with 25 µL of 10 ng µL−1 nitrophenol-d4 (internal standard, IS) and
extracted ultrasonically in 3–5 mL of methanol twice (15 min each). After
filtration and concentration, the final volume was roughly 500 µL
prior to HPLC/DAD-Q-ToF-MS analysis. An Agilent 1200 series HPLC equipped
with a Zorbax Eclipse Plus C18 column (2.1 mm×100 mm, 1.8 µm particle size, Agilent Technologies) was used to separate the target NACs
with an injection volume of 2 µL. The flow rate of the column was set
at 0.2 mL min−1, and the gradient separation was conducted with 0.2 %
acetic acid (v∕v) in water (eluent A) and methanol (eluent B). The
concentration of eluent B was 25 % for the first 3 min, increased to
100 % from 3 to 10 min, held at 100 % from 10 to 32 min, and then
decreased back to 25 % from 32 to 37 min. The identification and
quantification of NACs were determined with an Agilent 6520 Q-ToF-MS. The
Q-ToF-MS was equipped with a multimode ion source operating in electrospray
ionization (ESI) and negative (−) ion modes. All samples were analyzed in
full scan mode (40–1000 Da), and an acceptance criterion of ±10ppm
mass accuracy was set for compound identification and quantification. Then
selected samples were re-examined using the collision-induced dissociation (CID)
technique under identical chromatographic conditions. The MS∕MS spectra of
target [M–H]− ions provided m∕z data,
which were used for identifying NAC structures.

The extracted ion chromatograms (EICs) and Q-ToF MS∕MS spectra for
identified compounds in selected BB samples are provided in Fig. S1 in the
Supplement and in Fig. 1, respectively. The Q-ToF MS∕MS spectra
of standard and surrogate compounds used in this work are obtained from Xie
et al. (2017a) and provided in Fig. S2 for comparison. Table 2 provides the
formulas, standard and surrogate assignments, and proposed structures of the
identified NACs. Due to the lack of authentic standards, most of the NACs in
BB samples were quantified using surrogates in this work. In general, the
surrogate compound with similar molecular weight (MW) and/or structure was
selected for the mass quantification of each identified NAC. Since the
standard compound with hydroxyphenyl cyanate structure is not commercially
available, C8H7NO4 and C9H9NO4 were quantified
as 2-methyl-5-nitrobenzoic acid (C8H7NO4) and
2,5-dimethyl-4-nitrobenzoic acid (C9H9NO4), respectively; all
the identified NACs with MW >200Da were quantified as
2-nitrophloroglucinol (C6H5NO5). The mass quantification was
conducted using the internal standard method with nine-point calibration curves
(∼0.01–2 ng µL−1). The compounds corresponding
to each NAC formula (including isomers) were quantified individually and
added together for the calculation of mass contribution (%) to organic
matter (OM µg m−3) in each sample. The quality assurance and
control (QA∕QC) procedures applied for NAC quantification were provided in
Xie et al. (2017a). Field blank and background samples were free of
contamination for NACs. Average recoveries of standard compounds ranged from
75.1 % to 116 %, and the method detection limit ranged from 0.70 to
17.6 pg
(Table S2).

3.1 Light absorption of extractable OC

The average EC∕OC ratio, OC extraction efficiency, MAC365,
MAC550,
and Åabs of all samples grouped by experiment and fire phase are
shown in Table 1. Abbreviations for each sample group are also listed in the
table. The optical properties and bulk composition of the FL forest samples
were reported in Xie et al. (2017b). The average extraction efficiency for
all groups of BB samples is greater than 95 % (range 97.0±1.87 % to
99.5±0.33 %), and the light absorption exhibits strong wavelength
dependence, with average Åabs values ranging from 5.68±0.70
to 7.95±0.22. For each of the two NC forest experiments, the samples
collected during the flaming phase (NF1 and NF2) have significantly higher
(Student's t test, p<0.05) average EC∕OC ratios, MAC365, and
MAC550 and lower (p<0.05) Åabs than those collected
during the smoldering phase (NS1 and NS2). When combining the results from
the two NC forest experiments, the average MAC365 values for NC forest 2
are significantly (p<0.05) higher than NC forest 1, despite
having a comparable EC∕OC ratio (NF1=0.042±0.014 and
NF2=0.049±0.011, NS1=0.0098±0.0024 and NS2=0.0075±0.0026). Additionally, the average EC∕OC ratio of FF samples is
5–30 times higher than the NF and NS samples, while the average MAC365 and
MAC550 values of FF samples (1.13±0.15 and 0.053±0.023m2 g C−1)
are comparable to NS1 samples (1.10±0.11 and
0.054±0.015m2 g C−1) but lower than other NC forest
samples.

a Isomer 1; b standard compounds used for the quantification of
identified N-containing aromatic compounds; c standard compounds used
to estimate the light absorption of N-containing aromatic compounds.

High-temperature pyrolysis or intense flaming conditions are known to
increase the fraction of EC in the total carbonaceous aerosol emissions of
BB (Hosseini et al., 2013; Eriksson et al., 2014; Martinsson et al.,
2015; Nielsen et al., 2017). Several studies found that the light-absorbing
properties of BB OC could be parameterized as a function of the EC∕OC or
BC∕OA (organic aerosol) ratio, a measurement proxy for burn conditions
(McMeeking et al., 2014; Saleh et al., 2014; Lu et al., 2015; Pokhrel et al.,
2016), and inferred that the absorptivity of BB OC depended strongly on burn
conditions, not fuel type. In Xie et al. (2017b), significant correlations
(p<0.05) between MAC365 of methanol-extractable OC from BB and
EC∕OC ratios were observed only for samples with identical fuel type but
not for pooled samples with different fuel types, indicating that both burn
conditions and fuel types can impact the light absorption of BB OC. The
contradiction is possibly ascribed to different approaches used in
characterizing the light absorption of BB OC and different test fuel types
(Xie et al., 2017b). In the current work, we combined the sample
measurements from all three BB experiments and analyzed the correlations of
bulk MAC365 vs. EC∕OC. For the analysis, we removed one FL forest
experiment sample due to the extremely high EC∕OC ratio of 0.58 (burn 3,
Table S1). Generally, EC∕OC ratios are <0.4 for laboratory BB
(Akagi et al., 2011; Pokhrel et al., 2016; Xie et al., 2017b) and ≤0.1
for field BB (Aurell et al., 2015; Xie et al., 2017b; Zhou et al., 2017).
Thus, the burn condition of the FL forest burn 3 (Table S1) is
unrepresentative of laboratory BB simulations or field BB. In Fig. 2a, the
bulk MAC365 of methanol-extracted OC correlated significantly (p<0.05) with EC∕OC for each BB experiment. However, grouping these
sample measurements resulted in no correlation between MAC365 and the EC∕OC
ratio (Fig. 2b). Similar results were also observed for MAC550 vs. EC∕OC and Åabs vs. EC∕OC correlations (Fig. S3a–d). These results
show that BB BrC absorption depends on more than fire conditions, and
light-absorbing components can be formed at relatively low EC∕OC (e.g., tar
balls) from smoldering biomass combustion (Chakrabarty et al., 2010).

In this work, both the comparison of the flaming versus smoldering samples
for each NC experiment (Table 1) and the regressions of bulk MAC365
versus EC∕OC for individual burns (Fig. 2a) suggest that the light
absorption of OC from BB is strongly dependent on burn conditions when the
fuel type and ambient conditions are similar. The comparison of the FL
versus NC forest experiments (Table 1) and the relationship between bulk
MAC365 and EC∕OC for grouped measurements (Fig. 2b) indicate that the
burn conditions are not the only factor impacting BB OC absorption. The two
NC forest experiments were conducted in spring and summer, respectively,
with distinct ambient conditions (Table S1), and their average MAC365
values were significantly (p<0.05) different. This could be partly
ascribed to the fact that more semi-volatile organic compounds (SVOCs) will
partition into the gas phase in summer with higher ambient temperatures, and the SVOC is less
light-absorbing than OC with low volatility (Chen and Bond, 2010; Saleh et al.,
2014). However, if the relative abundance of EC and OC from BB emissions is
similar between the two NC forest experiments, the evaporation of SVOCs in
summer will lead to higher EC∕OC ratios, which is not observed in Table 1.
No previous study investigated the seasonal variation in BrC absorption from
BB with similar fuel type. Chen et al. (2001) found that the ambient
temperature might play a role in EC production from traffic by changing the
air density. We suspected that the BB samples from NC forest 2 combustion in
summer contained much stronger light-absorbing components than those from NC forest 1
combustion in spring, although the formation mechanism of these strong BrC
components is uncertain and merits further study. Therefore, the light
absorption of BB OC is influenced by factors other than burn conditions, and
EC∕OC ratios alone may not predict BB OC light absorption from burns with
varying fuel types and ambient conditions.

3.2 Identification and quantification of NACs

In the current work, 14 NAC chemical formulas in BB samples were
identified (Table 2) using the HPLC/DAD-Q-ToF-MS analysis, covering all the
NACs with high abundance and strong absorption in ambient and BB particles
reported in previous work (Claeys et al., 2012; Mohr et al., 2013; Zhang et
al., 2013; Chow et al., 2016; Lin et al., 2016, 2017). Their EICs
are provided in Fig. S1. The NAC structures corresponding to each chemical
formula were examined using MS∕MS data in Fig. 1. In Table S3, the averages
and ranges of relative mass contributions of identified NACs to OM are
provided by the BB experiment and burn condition. Here the OM mass was
calculated as 1.7×OC mass (Reff et al., 2009). In addition, the
average relative mass contributions of each NAC in BB samples are shown in
Fig. 3.

The three BB experiments have consistent mass contribution profiles (Fig. 3),
although they used different fuel types and were conducted in different
seasons. In Table S3, the BB samples collected during flaming periods (NF1
and NF2) contain significantly higher (p<0.05) average relative
mass contributions from total NACs to OM (tNACOM %: NF1 0.18±0.067 %, NF2 0.16±0.045 %) than those collected during
smoldering periods (NS1 0.055±0.026 %, NS2 0.023±0.0089 %). During the FL forest burn experiment, flaming and smoldering
phases were not separated for sampling, and the average tNACOM % is
0.13±0.059 %, which is between the tNACOM % of the flaming
and smoldering samples of the NC forest experiments. If we recalculate the
average tNACOM % for the NC forest experiments by combining the
flaming and smoldering sample data in each burn, the three BB experiments
(FL forest, NC forest 1, and NC forest 2) show similar average tNACOM % (0.11±0.017 %–0.13±0.059 %), and the average tNACOM %
across all samples in this work is 0.12±0.051 % (range 0.037 % to
0.21 %). This value is comparable to that observed at Detling
(∼0.5 %), United Kingdom, during winter, when domestic wood
burning is prevalent (Mohr et al., 2013). In the current work, most of the
NACs were quantified using surrogates, and their contributions to OM from BB
may change if authentic standards or different surrogates are used for
quantification. However, the three experiments might still have consistent
relative mass contribution profiles of NACs and similar average
tNACOM %, assuming burn conditions and fuel types have minor impact
on the OM∕OC ratio. As shown in Figs. S3e and 2c, tNACOM %
correlated (p<0.05) with EC∕OC for both individual burns and pooled
experimental data. Therefore, unlike the light absorption of
methanol-extractable OC, the formation of NACs in BB seems to depend largely on burn
conditions rather than fuel types and ambient conditions.

Among the 14 identified NAC formulas, C6H5NO4 and
C9H9NO4 have the highest concentrations (Fig. 3) in FL forest
and NC forest flaming-phase samples, accounting for 0.029±0.011 % to
0.037±0.011 % and 0.023±0.012 % to 0.049±0.016 %
of the OM, respectively (Table S3). In NC forest smoldering-phase samples,
C6H5NO4 has the highest mass contribution (NS1 0.024±0.0098 %, NS2 0.010±0.0027 %), followed by
C7H7NO4 (NS1 0.0087±0.0030 %, NS2 0.0043±0.0010 %) and C9H9NO4 (NS1 0.0052±0.0033 %, NS2
0.0047±0.0013 %) (Table S3). The C6H5NO4 was
identified as 4-nitrocatechol by comparing its MS∕MS spectrum (Fig. 1b) with
that of an authentic standard (Fig. S2b) in Xie et al. (2017a). The EIC of
C9H9NO4 exhibited three to four isomers (Fig. S1i), while only two
MS∕MS spectra (Fig. 1l, m) were obtained due to the weak EIC intensity for
compounds eluting at times ≥10min. The fragmentation patterns of
C9H9NO4 compounds (Fig. 1l, m) are different from that of
2,5-dimethyl-4-nitrobenzoic acid (reference standards with the same formula,
Fig. S2g) without the loss of CO2, suggesting that the
C9H9NO4 compounds identified in this work lack a carboxylic
acid group. Both MS∕MS spectra of the two C9H9NO4 isomers
reflect the loss of OCN (Fig. 1l, m), suggesting a skeleton of benzoxazole or benzisoxazole
or the existence of a cyanate (–O–C≡N) or isocyanate
(–N=C=O) group. Volatile organo-isocyanate structures (e.g.,
CH3NCO) were identified from anthropogenic biomass burning (Priestley
et al., 2018), and benzoxazole structures have been observed in pyrolyzed
charcoal smoke (Kaal et al., 2009). Giorgi et al. (2004) investigated the
fragmentation of 3-methyl-1,2-benzisoxazole and 2-methyl-1,3-benzoxazole
using a CID technique under different energy frames and found a loss of CO
but not OCN for both of them. In this work, four standard compounds,
including phenyl cyanate (C6H5OCN), benzoxazole
(C7H5NO), 4-methoxyphenyl isocyanate (CH3OC6H4NCO),
and 2,4-dimethoxyphenyl isocyanate ((CH3O)2C6H3NCO) were
analyzed using a gas chromatographer (Agilent 6890) coupled to a mass
spectrometer (Agilent 5975B) under electron ionization (EI, 70 eV) mode.
These compounds do not have a phenol structure and cannot be detected using
ESI under negative ion mode. The MS∕MS spectra of 4-methoxyphenyl isocyanate
and 2,4-dimethoxyphenyl isocyanate were obtained by using a modified method
(ESI at positive ion mode) for NAC analysis in this work. As shown in Fig. S4a and b,
the loss of OCN is observed for phenyl cyanate but not
benzoxazole. In Fig. S4c and d, the ions at m∕z 106 and 136 can be produced
from the species at m∕z 149 and 179 through the loss of CH3+CO or
H+NCO (43 Da). The MS∕MS spectra of 4-methoxyphenyl isocyanate and
2,4-dimethoxyphenyl isocyanate (Fig. S4e, f) confirmed the loss of
CH3+CO, and the loss of CH3 reflected the presence of a methoxy group. As
such, the C9H9NO4 compounds identified in this work are
expected to contain a phenyl cyanate structure.

C6H5NO3 (Fig. 1a) is identified as 4-nitrophenol using an
authentic standard (Fig. S2a). C7H7NO4 has at least two
isomers as shown in Fig. S1c that are identified as 4-methyl-5-nitrocatechol
and 3-methyl-6-nitrocatechol according to Iinuma et al. (2010) and Xie et
al. (2017a). Referring to the MS∕MS spectrum of 4-nitrocatechol (Fig. S2b),
the C6H5NO5 compound should have a nitrocatechol skeleton
with an extra hydroxyl group on the benzene ring. Like
C9H9NO4 (Fig. 1l, m), the loss of OCN was observed for the
fragmentation of C8H7NO4 in the MS∕MS spectra (Fig. 1f, g),
and a phenyl cyanate structure was proposed (Table 2). However, the
fragmentation mechanism associated with the loss of single nitrogen is
unknown and warrants further study. The C8H9NO4 identified in
this work should have several isomers (Fig. S1f), and two representative
MS∕MS spectra are provided in Fig. 1h and i. The first isomer of
C8H9NO4 has a dominant ion of m∕z 137, reflecting the loss of NO
and CH3. Comparing to the MS∕MS spectrum of 4-nitrophenol (Fig. S2a),
the first C8H9NO4 isomer might contain a methyl nitrophenol
skeleton with a methoxy group. The fragmentation pattern of the second
isomer of C8H9NO4 is similar to C7H7NO4, and
the molecule is postulated as ethyl nitrocatechol. C7H7NO5
has a similar fragmentation pattern to C6H5NO4 and
C7H7NO4 and is identified as methoxy nitrocatechol. For NC
forest burns, C10H7NO3 was only detected in flaming-phase
samples (Fig. 3). The MS∕MS spectrum of C10H7NO3 was subject
to considerable noise, although the loss of NO2 could be identified
(Fig. 1k). In Fig. 1n, the ion at m∕z 167 is attributed to the loss of two
CH3 from the [M–H]− ion of C8H9NO5, and the loss of
H+CO+NO is a common feature shared by several nitrophenol-like
compounds (Fig. 1b, c, e, i), so the C8H9NO5 compound was
identified as dimethoxynitrophenol. The MS∕MS spectra of
C10H11NO4, C10H11NO5,
C11H13NO5, and C11H13NO6 were characterized by
the loss of CH3 and/or OCN (Fig. 1o–t), indicating the existence of
methoxy and/or cyanate groups (Fig. S4). Although the exact structure of
these NACs cannot be determined, their functional groups on the benzene ring
were proposed in Table 2 from their fragmentation patterns.

In this work, three of the identified NACs, 4-nitrophenol, 4-nitrocatechol,
and methyl nitrocatechols, were commonly observed in BB emissions or
BB-impacted atmospheres (Claeys et al., 2012; Mohr et al., 2013; Budisulistiorini
et al., 2017). These compounds can also be generated from the
photooxidation of aromatic VOCs in the presence of NOx (Iinuma et
al., 2010; Lin et al., 2015; Xie et al., 2017a). Both BB and fossil fuel
combustion can emit a mixture of aromatic precursors (e.g., benzene,
toluene) for secondary NAC formation (Martins et al., 2006; Lewis et al.,
2013; George et al., 2014, 2015; Gilman et al., 2015; Hatch et al., 2015). Therefore, the NACs uniquely related to BB are needed to
represent BB emissions. In this work, the NAC formulas with molecular
weight <200Da (from C6H5NO3, 138 Da; to
C8H9NO5, 198 Da) were all identified in secondary organic
aerosol (SOA) generated from chamber reactions with NOx (Xie et al.,
2017a). However, the NACs from BB emissions and SOA formations with
identical formulas might have different structures. For example, the MS∕MS
spectra of C7H7NO5 and C8H9NO5 from BB in this
work and aromatic VOCs and NOx reactions in Xie et al. (2017a) had distinct
fragmentation patterns (Fig. S5). In Xie et al. (2017a), the
C8H7NO4 and C9H9NO4 generated from
ethylbenzene and NOx reactions might have fragile structures and their
MS∕MS spectra were not available. In this work, C8H7NO4 and
C9H9NO4 from BB emissions are more stable and are supposed to
have a phenyl cyanate structure. Among the four NAC formulas with MW
>200Da identified in this work (Table 2),
C10H11NO4 was also observed as
5-methoxy-4-nitro-2-(prop-2-en-1-yl)phenol in SOA from reactions of methyl
chavicol and NOx (Pereira et al., 2015), which cannot be assigned to
the C10H11NO4 from BB emissions in this work. Compared to the
NACs in aromatic VOCs and NOx SOA (Iinuma et al., 2010; Lin et al.,
2015; Xie et al., 2017a; Pereira et al., 2015), the structures of NACs from
BB in this work were characterized by methoxy and cyanate groups. The
methoxyphenol structure is a feature in polar organic compounds from BB
(Schauer et al., 2001; Simpson et al., 2005; Mazzoleni et al., 2007). The
cyanate group was rarely reported in gas- or particle-phase pollutants from
BB, which might be a missed feature of BB NACs. Vähä-Savo et al. (2015) found that cyanate could be formed during the thermal conversion
(e.g., pyrolysis, gasification) of black liquor, which is the waste product
from the kraft process when digesting pulpwood into paper pulp and composed
by an aqueous solution of mixed biomass residues. According to Table 2 and
Fig. 3, the NACs containing methoxy and/or cyanate groups are predominately
generated during the flaming phase in the two NC forest experiments. Before
using these compounds as source markers for BB NACs, additional work is
warranted to understand their exact structures and lifetimes in the
atmosphere. The quantification of these compounds might also be subject to
high variability due to the usage of surrogates.

3.3 Contribution of NACs to Abs365

For each sample extract, individual NAC contributions to Abs365
(Abs365,NAC %) were calculated using their mass concentrations (ng m−3)
and the MAC365 values of individual compound standards
(MAC365,NAC), as applied in Zhang et al. (2013) and Xie et al. (2017a).
Here, the MAC365,NAC value is OM based with a unit of m2 g−1.
Each NAC formula was assigned to an authentic or surrogate standard compound
to estimate the contribution to Abs365 of extracted OM (Table 2).
Except for the NACs with a phenyl cyanate structure, the standard compounds used
for the NAC absorption calculation and mass quantification were the same
(Table 2), and their UV–Vis spectra were obtained from Xie et al. (2017a)
and are shown in Fig. S6a. The UV–Vis spectra of three standard compounds with
cyanate or isocyanate groups are given in Fig. S6b, and none of them has
absorption in the range from 350 to 550 nm. As such, the NACs with cyanate
groups identified in this work were supposed to have no contribution to bulk
Abs365. Details of the method for Abs365,NAC % calculation are
provided in Xie et al. (2017a) and the MAC365,NAC values for identified
NAC formulas in this work are listed in Table S4. Since the standard
compounds used in this work have no absorption at 550 nm, the identified
NAC contributions to Abs550 were expected to be 0. The average and
ranges of Abs365,NAC % in BB samples are listed in Table S5. For
simplicity, the average Abs365,NAC % values in the five groups of BB samples
(FF, NF1 and 2, NS1 and 2) are stacked in Fig. 4.

In general, the average contributions of total NACs to Abs365
(Abs365,tNAC % 0.087±0.024 % to 1.22±0.54 %) were
3–10 times higher than their average tNACOM % (0.023±0.0089 %
to 0.18±0.067 %) in BB samples (Tables S5 and S3), indicating that
the identified NACs with contributions to Abs365 (not including those
with cyanate groups) are strong BrC chromophores. Similar to the NAC mass
contributions and compositions, the samples collected during flaming periods
(NF1 and NF2) had a significantly higher (p<0.05) average
Abs365,tNAC % (NF1 1.21±0.38 %, NF2 0.42±0.15 %)
than those collected during smoldering periods (NS1 0.72±0.27 %,
NS2 0.087±0.024 %); Abs365,tNAC % correlated (p<0.05) with EC∕OC for both individual burns (Fig. S3f) and pooled
experimental data (Fig. 2d). C6H5NO4 (0.037±0.0080 % to
0.31±0.11 %) and C7H7NO4 (0.029±0.0051 % to
0.27±0.12 %) have the highest Abs365,NAC % among the
identified NACs across all the three BB experiments (Table S5). The average
Abs365,tNAC % values here are comparable to those obtained for
atmospheric particles in Germany (0.10±0.06 % to 1.13±1.03 %) (Teich et al., 2017) and Detling, United Kingdom (4±2 %)
(Mohr et al., 2013), but are more than 10 times lower than those from chamber
reactions of benzene (28.0±8.86 %), naphthalene (20.3±8.01 %), and m-cresol (50.5±15.8 %) with NOx (Xie et al.,
2017a). Lin et al. (2016, 2017) calculated the absorbance fraction
contributed by NACs in BB OC based on signal peaks at particular retention
times in HPLC/PDA (photodiode array) spectrophotometry chromatograms and
attributed a large portion (up to or greater than 50 %) of the solvent
extract absorption to a limited number of NACs with MW mostly lower than
500 Da. However, the absorbance signals in HPLC/PDA chromatograms are
composed by a mixture of light-absorbing compounds due to coelution, and
some of them are not NACs or even cannot be ionized with ESI. In this study,
standards or surrogates were used to calculate the absorption for individual
NAC molecules. These different approaches gave different results. Di
Lorenzo et al. (2017) studied the absorbance as a function of the molecular size
of organic aerosols from BB and concluded that the majority of aqueous
extract absorption (λ=300nm) was due to compounds with MW
greater than 500 Da and a carbon number greater than 20. In this work, less
than 2 % of the BrC absorption in BB aerosols at λ=365 was
ascribed to the identified NACs with a MW range of 138 to 254 Da, of which
the contribution at a longer wavelength (λ=550nm) was expected
to be 0. Future work is needed to identify high-MW light-absorbing compounds
in BB aerosols to apportion a greater fraction of BrC absorption in BB
aerosols.

The comparisons of light-absorbing properties (MAC365,
MAC550, and Åabs) of BB OC with EC∕OC in this study
show that burn conditions are not the only factor impacting BrC absorption. Other
factors like fuel type or ambient conditions may also play important roles
in determining BrC absorption from BB. It may be impractical to predict BrC
absorption solely based on EC∕OC ratios in BB emissions from different fuels
or over different seasons. The present study identified 14 NAC
chemical formulas in BB aerosols. The average tNACOM % values of the FL
forest, NC forest 1, and NC forest 2 (flaming and smoldering samples were combined)
experiments were 0.13±0.059 %, 0.13±0.067 %, and 0.11±0.017 % by weight, respectively, and the NAC composition was also
similar across the three BB experiments. Most of the NAC formulas
identified in this work were also observed in simulated SOA generated from
chamber reactions of aromatic VOCs with NOx, but the same NAC formula
from BB and SOA could not be assigned to the identical compound. In this
work, the structures of NACs from BB were characterized by methoxy and
cyanate groups, which were predominately generated during the flaming phase
and might be an important feature for BB NACs. More work is warranted to
understand their exact structures and lifetimes. The average tNACOM %
and Abs365,tNAC % of the flaming-phase samples were significantly
higher (p<0.05) than those of smoldering-phase samples in the two
NC forest BB experiments. Unlike the bulk MAC365 and MAC550,
tNACOM % and Abs365,tNAC % correlated (p<0.05) with
EC∕OC for both individual burns and pooled experimental data, suggesting
that burn conditions are an important factor in determining NAC formation
in BB. Except for the compounds with cyanate groups, the NACs identified in this
work are likely strong BrC chromophores, as the average contributions of
total NACs to bulk Abs365 (0.0.087±0.024 % to 1.22±0.54 %) are 3–10 times higher than their average mass contributions to OM
(0.023±0.0089 % to 0.18±0.067 %). However, more
light-absorbing compounds from BB with high MW need to be identified to
apportion the unknown fraction (>98 %) of BrC absorption.

MX and ALH designed the research. MX and XC performed the experiments. ALH and
MDH managed sample collection. MX analyzed the data and wrote the paper with
significant contributions from all co-authors.

This research was supported by the National Natural Science Foundation of
China (NSFC, 41701551), the State Key Laboratory of Pollution Control and
Resource Reuse Foundation (no. PCRRF17040), and the Startup Foundation for
Introducing Talent of NUIST (no. 2243141801001). We would like to
acknowledge Brian Gullett, Johanna Aurell, and Brannon Seay for assistance
with laboratory biomass burning sampling. This work was funded by the U.S.
Environmental Protection Agency.

We did a comprehensive work on understanding the composition and structural information of N-containing aromatic compounds (NACs) and their contributions to organic matter and bulk extract absorption of biomass burning (BB) aerosols. Some NACs with methoxy and cyanate groups specific to the BB were identified. The general implication is that the formation of NACs during BB might depend largely on burn conditions and is less impacted by fuel types and/or ambient conditions.

We did a comprehensive work on understanding the composition and structural information of...